Lab 11 - File IO and Eclipse | CS 163/4: Java Programming (CS 1) | CS 163/4: Java Programming (CS 1) | CS 163/4: Java Programming (CS 1) HomeTopicsResourcesPoliciesGradingLabsPracticals | Canvas CS 163/4: Java Programming (CS 1) Computer Science Virtual Help Desk InstructionsCS Course SelectionHomeTopicsResourcesPoliciesGradingLabsPracticalsCanvas Lab 11 - File IO and Eclipse Introduction to Eclipse What is Eclipse? CS163/4 uses the Eclipse Integrated Development Environment (IDE) for developing Java code. Eclipse is favored as a premier tool in academia and industry. A standard installation of Eclipse is available on all CS Department Linux computers for use by CS163/4 students. As a default, it will be assumed that students use this version on CS Department Machines. Basics of how to use the Eclipse IDE productively will be covered somewhat in lecture and more extensively in course recitations. Lab machines are currently using the Oxygen version of Eclipse, which is version 4.7.2. Eclipse documentation is found here. Eclipse can also be downloaded for free and installed on most platforms. Eclipse is written entirely in Java, so it will operate properly on most any platforms that support Java. This includes Linux, Solaris, Mac OS X and properly configured Windows platforms. It is assumed that many students will find it possible to install and use Eclipse on their own computers, and the instructor and GTAs will try to help informally. However, successful installation and use of tools such as Eclipse on your own machine demands some understanding of how to install and use new software on that platform, and ultimately it falls to the individual student to either succeed at such an effort or to use the resources already provided by the CS Department. The help that can be offered by the instructor and GTAs is by necessity limited. If you decide to run Eclipse on your system, make sure you download and install the Java Development Kit (JDK) first from here. The current version of Java on department systems is 1.8 (Version 8). After installing Java, download and install Eclipse from here. Select the version called Eclipse IDE for Java Developers. In both cases make sure you select the correct operating system. Note: as of this update, Eclipse has been having issues with Version 14 of java. You may want to install version 1.8 and not 14. Important From this point onward all coding assignments will be done by downloading files from zybooks, editing them in eclipse and uploading them back to zybooks for grading. Introduction to Lab Today we will be discussing file input and output, or File I/O. File I/O allows us to use external files to read in data for our programs to use, including text files, csv (Comma Separated Value files), image files, and more. In addition to being able to read from files, File I/O also allows us to write to files and create permanent file output for our users. File I/O Java provides strong I/O support. There are multiple ways to do I/O in Java, however, in this lab we will be focusing on two main ways. One way for input and another for output. There are some common similarities in regard of how to do input and output. Specifically, error handling. Input (File Objects) For this lab we will be inputting files with the use of File Objects. File Objects are made just like any other object that we have seen before. The constructor for the File object takes a String that represents a path to a file that it wants to do input with. The path of the file can be local or absolute. An example of a local path would be, if a file resided in a project folder in eclipse. Since this file is local to the project, the path name for this file would be the file name itself. However, if the file resided somewhere else, the absolute path would need to be given. An important note about the file path is that if the file does not exist/cannot be found, Java will throw an exception (more on this later). Below is an example between local and absolute path for a file named Example.txt. Local Path:
File fileObj = new File("Example.txt");
Absolute Path:
File fileObj = new File("~/users/Documents/Example.txt"); Once a File Object is initialized, methods can be used from its class to do input manipulation. However, we will not be using those methods in this lab, instead we will be using a Scanner. To learn more about File Objects, visit this link: File Java Doc File and Scanner Objects File and Scanner Objects work well together. As we have seen before, we can use a Scanner to parse input from a keyboard and save that input into variables. Scanners can also be used to parse through files. In order to do this, a Scanner Object must be made with an initialized File Object in its constructor. By having this, the Scanner’s methods such as .next() and .nextInt() can be used for parsing a file. Shown below is two ways to initialize a File and Scanner Object to parse a file. Both have the same result, one just has less code. Option 1:
File fileObj = new File("Example.txt");
Scanner fileScanner = new Scanner(fileObj);
Option 2:
Scanner fileScanner = new Scanner(new File("Example.txt")); Output (PrintWriter Objects) For this lab, we will be outputting to files with the use of PrintWriter Objects. Similarly, to File Objects, PrintWriter Objects are initialized the same way. The PrintWriter constructor takes a local or absolute path to a filename. However, unlike the File Objects if the filename passed to the constructor does not exist, that file will be created in the path given. PrintWriters have many methods that can output to a file. The most common ones being .print() and .println(). Notice that these are similar to System.out.print() and System.out.println(). These function the same, however, instead of outputting to the console it outputs to a file. An important part with PrintWriters is closing them. This must done after using the PrintWriter in order to flush the buffer. Below is an example of using a PrintWriter. To learn more about PrintWriters, visit this link: PrintWriter Java Doc PrintWriter output = new PrintWriter(new File("Example.txt"));
output.println("Hello, World!");
output.close(); //Closing the PrintWriter Try/Catch When using File I/O there is some overhead in exception handling. When reading or writing to a file, exceptions can be thrown by Java. You have more than likely seen this before with exceptions like “NullPointerException” or “ArrayIndexOutOfBounds”. This happens in Java when the program runs into a problem that disrupts the normal flow of the program. When this happens, Java does not have a direct way to fix it and therefore the program terminates followed by a message to the console of what went wrong. In File I/O, we have to take this into account. This is mainly due to the exception “FileNotFoundException” can occur especially with File Objects (Exceptions also occur in PrintWriter constructors). The way to deal with this is to surround the code that has a potential for an exception with a try/catch block. Try/Catch blocks work by putting the code that could cause an exception into the try portion of the block. This tells Java to try this code but knows that it may throw an exception. If the program does throw an exception, then the catch portion of the block can catch the exception and do something about it. Typically, the stack trace is outputted to the console from the catch block. However, you could write code in the catch block that could execute. Below is an example. File Objects 1
2
3
4
5
6
7
8
9
public static void main(String[] args){
try{
File fileObj = new File("Example.txt");
Scanner fileScanner = new Scanner(fileObj); //Exception could occur if the file does not exist
} catch(FileNotFoundException e){
e.printStackTrace();
}
} PrintWriter Objects 1
2
3
4
5
6
7
8
9
10
public static void main(String[] args){
try{
PrintWriter output = new PrintWriter(new File("Example.txt")); // Exception could occur in constructor
output.println("Hello, World!");
output.close();
} catch(FileNotFoundException e){
e.printStackTrace();
}
} ZyBooks Assignment Part 1 readFile(String filename) In this method you are passed a String with name of a file. This method will read the file in line by line and store each line in a String array. This String array is then returned. An example is shown below. File Contents: Purple Rain by Prince I never meant to cause you any sorrow I never meant to cause you any pain I only wanted one time to see you laughing I only want to see you laughing in the purple rain String Array Contents: [0]: Purple Rain by Prince [1]: I never meant to cause you any sorrow [2]: I never meant to cause you any pain [3]: I only wanted one time to see you laughing [4]: I only want to see you laughing in the purple rain In order to do this, you will need a String array, an int that keeps track of how many lines there are in a file, a File Object, and a Scanner Object. Assume that the String array holds a max of 1000 elements. Notice that there are three lines of code written at the bottom. DO NOT MODIFY THESE LINES. This code is copying your String array into a new array with the same amount of elements as there are lines in the file. Since this code is written for you, it assumes that your String array is named lines and that the int that keeps track of the amount of lines is named lineCounter. You will loop through and parse this file with the use of the Scanner and File Object and store each line in the String array while also counting each line. Hints How can a Scanner be used to know when the file ends? You will need a try/catch block. Part 2 reverseFile(String[] parsedFile, String filename) This method takes a String array that has lines of a file in it and a filename. This method will write the contents of the String array in reverse order to the file passed. The writing will be done with a PrintWriter. Here is an example. String Array Content: [0]: Purple Rain by Prince [1]: I never meant to cause you any sorrow [2]: I never meant to cause you any pain [3]: I only wanted one time to see you laughing [4]: I only want to see you laughing in the purple rain Files Contents: I only want to see you laughing in the purple rain I only wanted one time to see you laughing I never meant to cause you any pain I never meant to cause you any sorrow Purple Rain by Prince Hint You will need a try/catch block. Part 3 logFile(String[] parsedFile, String filename) This method takes a String array that has lines of a file in it and a filename. This method will write contents of the String array to a file. However, It will only write the lines that contain “LOG”. Other lines will be skipped over. Here is an example. String Array Content: [0]: LOG username: happy_cat31 [1]: password: MrMuffins1234 [2]: LOG username: sad_panda [3]: password: BambooLover83 Files Contents: LOG username: happy_cat31 LOG username: sad_panda Hints What String method could help determine if a file contains “LOG”? String Java Doc You will need a try/catch block. Computer Science Department 279 Computer Science Building 1100 Centre Avenue Fort Collins, CO 80523 Phone: (970) 491-5792 Fax: (970) 491-2466 Department Courses Degrees Faculty & Staff Prospective Students CS 163/4: Java Programming (CS 1) Computer Programming in Java: Topics include variables, assignment, expressions, operators, booleans, conditionals, characters and strings, control loops, arrays, objects and classes, file input/output, interfaces, recursion, inheritance, and sorting. Apply to CSU Contact CSU Disclaimer Equal Opportunity Privacy Statement © 2019 Colorado State University - Computer Science Department, Fort Collins, CO 80523
